This invention relates to a method for protecting motors and for predicting motor life, and, more particularly, to controlling motor overload and predicting motor life based on the prior history and experience of the motor.
One of the chief factors limiting induction motor life is winding insulation degradation caused by repeated or excessive overheating. Manufacturers typically specify two winding temperature limits: one for steady state and one for transients. Historically, this is done because it is difficult for prior overload protection devices to estimate or model the winding temperature very accurately during transients.
A motor operating under normal conditions is expected to last a certain number of years. However, circuit breakers supplying power to motors are generally either tripped (i.e., opened) at the first sign of an overload condition, thereby causing operational nuisances in the case of transients, or allow the motors to overheat to a certain set point before tripping. In the latter case, since overloads are not controlled (i.e., for example, overloads once an hour are treated the same as overloads once a year), there is a wide distribution in the lifetime of motors because each overload contributes to an incremental loss of the motor's life expectancy.
For a motor to last out its life expectancy, a lifetime control is needed. For example, if a motor has a life expectancy of 40 years, generally it should use up 1/40 of its life each year. If overloads on the motor in one year cause more than 1/40 of its life to be used, an additional annual life must be gained from either the past or the future. If less than 1/40 of its useful life is used in a year then following years may be allowed to use more than 1/40 of its life in order to keep the motor on its lifetime schedule.
In motor operation, it would be useful to determine how much overload the motor should be allowed to withstand, and for how long, in order to keep the motor on its lifetime schedule. Presently there are several ways of detecting motor overloads and protecting motors against overheating. Bimetallic switches, or bi-metals, have been used in the current circuits of motors for many years to estimate motor heating and to trip the power circuit to the motor on overload conditions. However, bi-metals have a relatively large margin of error, a poor transient response and do not take into account previous life history of the motor. Resistive temperature detectors are often embedded in the windings of larger motors and supply currents proportional to temperature to motor protection equipment. Both bi-metals and resistive temperature detectors have short duration memories and operate on fixed temperature limits. Once a motor has cooled down from an overload situation, that event has no more effect on the overload detector. Therefore, except during this cooling down period, each overload is treated independently of each other overload even though each overload is a factor in determining the life expectancy of the motor.
Accordingly, it is an object of the present invention to protect a motor from the effects of overloading by preventing any overload from reducing the life of the motor to less than its original planned life.
Another object of the present invention is to protect a motor from the effects of overloading while eliminating overprotective nuisance circuit breaker trips.
Still another object of the present invention is to provide an indication of the expected remaining useful life of a motor.
Yet another object of the present invention is to prevent catastrophic and irreversible damage to a motor due to insulation deterioration.